Magnetic field sensors including a magnetic field sensing element, or transducer, such as a Hall Effect element or a magnetoresistive element, are used in a variety of applications to detect aspects of movement of a ferromagnetic article, or target, such as proximity, speed, and direction. Applications using these sensors include, but are not limited to, a magnetic switch or “proximity detector” that senses the proximity of a ferromagnetic article, a proximity detector that senses passing ferromagnetic articles (for example, magnetic domains of a ring magnet or gear teeth), a magnetic field sensor that senses a magnetic field density of a magnetic field, and a current sensor that senses a magnetic field generated by a current flowing in a current conductor. Magnetic field sensors are widely used in automobile control systems, for example, to detect ignition timing from a position of an engine crankshaft and/or camshaft, and to detect a position and/or rotation of an automobile wheel for anti-lock braking systems.
Magnets, in the form of a permanent magnet, or magnetically permeable structures, sometimes referred to as concentrators or flux guides, are often used in connection with magnetic field sensors. In applications in which the ferromagnetic target is magnetic, a magnetically permeable concentrator or magnetic flux guide can be used to focus the magnetic field generated by the target on the magnetic field transducer in order to increase the sensitivity of the sensor and, allow use of a smaller magnetic target, or allow the magnetic target to be sensed from a greater distance (i.e., a larger airgap). In other applications in which the ferromagnetic target is not magnetic, a permanent magnet, sometimes referred to as a back bias magnet, may be used to generate the magnetic field that is then altered by movement of the target.
If a back bias magnet is used, the magnetic field sensing elements used in an application for detecting a target are placed within the magnetic field formed by the back bias magnet. Thus, the magnetic field sensing elements detect changes in bias magnetic field caused by movement of the target.
Generally, back bias magnets, on their own, do not produce a uniform magnetic field across the area where the magnetic field sensing elements are positioned, particularly over temperature. If the magnetic field sensing elements are placed within a non-uniform magnetic field, each of the sensors may be subject to a different DC offset. Compensating for the different DC offsets requires additional circuitry or algorithms that can increase the cost of the magnetic field sensor. Thus, concentrators are sometimes used in conjunction with back bias magnets to reduce the non-uniformity of the bias field. However, concentrators add cost to the magnetic circuit and can reduce the minimum usable air gap.